This web page describes a 1-month "pilot
project" of observations of white dwarf (WD) stars in a
search for exoplanet transits. The goal is to explore
ways of using amateur and professional observers in a way
that most efficiently produces an assessment of the fraction
of white dwarfs exhibiting transits by Earth-size planets in
the habitable zone. If no transits are found during
the 1-month pilot study, allowing for an evaluation of an
upper limit to their presence, such a result can be useful
in planning a comprehensive professional search for WD
transits. Amateur telescopes are suitable for this project
so it is anticipated that most observations will come from
the community of amateurs with experience observing
exoplanet transits (of main sequence stars). Any WD
exoplanet in the habitable zone will orbit with a very short
period (4 to 30 hours), will have very short transit lengths
(a few minutes) and will produce very deep transits
(complete eclipse possible for central crossing). Such
transits would be easy for amateurs to detect for stars as
bright as typical known transiting exoplanet stars, V-mag 10
- 13, but only a few WDs are this bright; the faintness of
WDs, and the short transit times of exoplanets that are of
interest, means that only advanced amateurs with prior
experience in observing exoplanet transits are being
recruited for this pilot study. This web page will be the
home site for an archive of light curve submissions, and
links will be included for web pages devoted to specific WDs
when they have light curves. The 1-month observing period is
set for September, 2011.

Both images and the
animation were created for PAWM by Manuel Mendez
(Spain).

This proposed "pilot project" endeavors to demonstrate that
advanced amateur astronomers are capable of detecting transits of
exoplanets orbiting white dwarfs in sufficent numbers to provide a
constraint on models for the presence of such exoplanets. If WD
exoplanets in close-in orbits are common, then an important
opportunity will exist for detecting Earth-sized exoplanets in
habitable zones. Such exoplanets will be comparable in size to
their WD parent star which means they can produce deep transits,
possibly totally eclipsing the star for a central crossing. Deep
transits help observability, all other things being equal, but two
other things render the WD transit search difficult: 1) the
population of WDs are faint due to their extremely low absolute
brightness (~ 0.01% of main sequence stars), and 2) transit
lengths are short due to the small size of the WD star and this
means only a few minutes worth of observations can be used to
establish the presence of a transit. The PAWM is therefore
challenging for amateurs using modest aperture telescopes, but it
is possible for those with experience observing exoplanet transits
(of main sequence stars). There is sufficient scientific merit in
the search for exoplanets in WD habitable zones that serious
thought deserves to be made for the funding of a comprehensive survey with a
network of professional telescopes that are optimum for the
search. One of the main reservations for funding such a survey is
the matter of whether or not planets exist in close-in orbits
around WDs. They can either migrate inward after the expansion to
the red giant phase and subsequent explosive shedding of an outer
layer, or planets can form in close-in orbit after this
cataclysmic event. Whether either of these processes occur
constitutes the most important unknown in the argument for
investing in a professional search for WD exoplanets. Amateurs can
provide a constraint on this matter by observing the brightest WDs
in search of at least one transit. A single transit, confirmed by
subsequent observations of it, could not only establish the
feasibility of a comprehensive professional search but it might
possibly constitute the first actual detection of an Earth-sized
exoplanet in a star's habitable zone!

The proposed pilot project may produce nothing, or it may produce
an important discovery. The chance of "success" in PAWM detecting
a transit is probably low, but the payoff for such a success is
unquestionably high. Even if no transits are detected an important
constraint will be made on the likely success of a professional
search. A
secondary goal of the PAWM and any follow-up professional survey
is the possibility of discovering eclipsing binaries consisting of
two white dwarfs. Any such discoveries could contribute to the
mass-radius relation for white dwarfs as well as contributing to
an understanding of how a WD binary might merge to produce a Type
Ia supernova.

The pilot project
of observations is scheduled to last one month, during September,
2011. Observers are encouraged to work in teams of at least two so
that any transit feature seen by one observer can be corroborated.
Because of the faintness of most WDs the observations will be
challenging, and I think previous experience with observing known
exoplanet transits is an essential requirement for attempting to
participate in this project. Although I anticipate that most
observers for PAWM will come from the 60 or so amateurs who have
experience with exoplanet transit observing I also welcome
participation by university students with access to larger
apertures and better CCDs, perhaps under the supervision of a
professional astronomer. I also welcome contributions, both
observations and advice, by professional astronomers interested in
this subject.

I will maintain an archive of WD observations submitted to me via
e-mail attachments, similar to the way it was done for the Amateur
Exoplanet Archive (AXA) project. I think it will be adequate for
observers to submit only their light curve, but I will also accept
a data file in a format described below.

Professional guidance for this pilot project will be provided by
Prof. Eric Agol (University of Washington, Seattle). Any other
astronomer with a suggestion for how PAWM should be conducted are
welcome to contact me (e-mail at bottom). Background reading for
this project can be found at New Scientist magazine (2011
July 2 issue, page 37) and an Astrophysical Journal Letters
article by Eric Agol, that is also available as an arXiv document,
link. Some Basic Background Related to Observing Strategy

There are ~ 20,000 known white dwarfs (from the ~60% of sky that
avoids low galactic latitudes). Of these only ~ 168 are brighter
than V-mag = 14.0; another 305 are within the range V-mag = 14
to15, and 742 are between V-mag = 15.0 and 16.0. In other words,
473 WDs have V-mag < 15, and 1215 have V-mag < 16.0. These
abundance statistics were provided by Eric Agol's amateur
astronomer associate Howard Relles and are based on a catalog
compiled by Rowell and Hambly (2011), link, as well as the VizieR
database (mostly from McCook, 2008). The Villanova White
Dwarf Catalog contains photometry and distance information, and it's
user-friendly: link.
The next figure illustrates that transits of WDs are likely to
be very deep. Note that an Earth-size planet with a central
crossing would produce a 50% flux drop (depth of ~ 750 mmag).

The following figure shows the kind of light curve (LC)
that's possible with a 14-inch telescope observing unfiltered
with 20-second exposures.Figure 2. Example of a light curve for a V-mag = 15.1
white dwarf using a 14-inch Meade telescope (M14) with a "clear
with blue-blocking" filter (Cbb) at the Hereford Arizona
Observatory (HAO) second dome. Individual image sxposure times
were 30 seconds. The model includes an arbitrarily placed transit
with 750 mmag depth and 4-minute length, to illustrate what an
Earth-sized exoplanet could produce for a central crossing. The
small "+" symbols correspond to individual images, and the red
circles are 11-point averages. The individual images exhibit an
RMS with respect to themselves of 29 mmag, and with respect to a
straight line fit of 28 mmag. The most important source for
scatter is "thermal noise" (associated with the CCD); Poisson
noise and scintillation are unimportant.

Since we're looking for very deep transits, in the several hundred
mmag range, this LC shows that a 14-inch telescope can produce
useful data for WDs as faint as V-mag = 15. How many WDs this bright
can be observed during the pilot project? There are ~ 450 WDs in the
entire sky this bright, but fewer are observable during a 1-month
observing interval from the northern mid-latitudes. I estimate
that ~ 150 WDs brighter than V-mag 15.0 are observable using amateur
hardware during this September's pilot project.

Target Selection and Observer Coordination

The best targets for mid-September observing have RA ~ 23:30. At
mid-latitudes stars at this RA that pass overhead can be observed
for ~10.0 hours. I propose to maintain a list of bright WDs near
this RA and with DE (declination) values between +25 to +50 degrees.
Observers are encouraged to select a target star in coordination
with other observers. Observers with a small telescope aperture will
have to settle for a bright WD, whereas observers with a large
aperture are encouraged to select a faint WD. The ideal observing
session is with 4 observers, 2 in Europe and 2 in USA, observing the
same WD target. This will provide continuous converage for a light
curve that could be 18 hours long (in September). This four-some
strategy allows corroboration of any interesting features, provided
all had clear skies. A two-some strategy is almost as valuable. It
could consist of two observers at a similar longitude (allowing
feature corroboration) or at widely spaced longitudes (providing
long continuous coverage). Finally, a single-observer strategy can
be employed; this may be best for an observer with an especially
large telescope aperture for it will provide a sampling of the very
faintest WDs.

I will suggest a short "hot target" list that everyone is invited to
use for selecting a target. When a target has been observed
sufficiently (e.g., 20 to 30 hours) I will drop it from the "hot"
list. Professional astronomer Eric Agol will be consulted in
creating the "hot" target list because his interests are focused on
WDs that are more likely to harbor exoplanets in the habitable zone
(medium to cool WDs).

Prof. Eric Agol will provide overall scientific guidance to this
project. He will take the lead in assessing the scientific value
of how this pilot project is conducted. Howard Relles, an
amateur astronomer associate, will provide support for such an
assessment. I expect that Prof. Agol will decide at the end of the
observing month whether a scientific publication of results is
warranted, and if he writes such a paper he will be lead author.
Co-authorship will be based on total number of data submission hours
as well as an estimate of data quality. Since Prof. Agol has
expressed interest in seeking funding for a large observing project
of professional telescopes I suspect that he will some day use the
results of this pilot study to support any proposed follow-on
project.

Expected Comprehensiveness of PAWM

One of the goals for PAWM is to establish a relationship between
specific telescope systems and the faintest WDS for which useful
monitoring can be achieved. Already with just three weeks of
exploratory WD light curves in the PAWM archive it is clear that a
12-inch telescope can produce useful LCs for a star having V-mag ~
16 (and probably fainter), provided the observer has experience with
exoplanet transit observing. See for example the excellent LC of a
15.6 mag star by Gregor Srdoc at link.
The following plots summarize what has been learned so far.

Figure. 3 Left:Preliminary result showing the faintest
V-mag's for which useful LCs can be obtained versus telescope
aperture. Figure. 3 Right:The number of known WDs brighter than
V-mag values.

For example, it appears that a 14-inch telescopes in use by
experienced observers are capable of producing useful LCs for more
than 2500 WDs.

Observer Sign-Ups

Here's a map of locations for observers who have signed-up for PAWM,
so far.

Questions keep coming, and they deserve to be answered in a way that
everyone interested in PAWM can access, and comment on as necessary.
Most people won't want to be burdened by that material so I'm
creating a "catch-all" web page for that material: Additional PAWM Supporting Material
("catch-all" web page).

Weekly Status Update E-mails

I will send e-mails at weekly intervals (Fridays) to everyone who
has signed-up to be an observer or "follower" of PAWM.

Apropos Quote

"When we are exploring the universe and looking for
evidence of life, either we may look for things that are
probable but hard to detect or we may look for things that are
improbable but easy to detect. In deciding what to look for,
detectability is at least as useful a criterion as probability.
Primitive organisms such as bacteria and algae hidden
underground may be more probable, but freeze-dried fish in orbit
are more detectable. To have the best chance of success, we
should keep our eyes open for all possibilities."
Freeman Dyson, Atlantic Monthly, 1997 November.